Press forming machines are used for punching press, wire drawing, die forging, injection molding and the like. A press forming machine having a stationary die and a movable die is generally used. A vertical press forming machine has a lower support stand, a plurality of support pillars supported at the lower support stand, an upper support plate held by the support pillars, and a slide plate capable of reciprocating along the support pillars between the lower support stand and the upper support plate and having a forming space between the lower support stand and itself. In the forming space, a stationary die is provided on the lower support stand, a movable die is provided on an undersurface of the slide plate, and a work piece is formed between the stationary die and the movable die. The slide plate is normally in a plane form, and is moved up and down by a drive mechanism. It is desirable to carry out forming by moving the movable die while keeping the movable die in desired positional relationship with respect to the stationary die, for example, while keeping the movable die horizontal.
Depending on the shape of a work piece subjected to press forming, an offset load occurs to the dies, and the positional relationship of the stationary die and the movable die or the slide plate is not maintained horizontal. It is proposed to keep the slide plate horizontal by controlling driving forces from the driving sources to keep synchronism among a plurality of driving sources, when a plurality of driving sources for driving the slide plate are installed in a press forming machine.
However, the work piece formed by press forming has a complicated shape such as a three-dimensional shape, and therefore not only the magnitude of the force exerted on the slide plate during forming changes with the proceeding of the forming, but also the location on which the force is exerted moves with the forming.
Reaction forces exerted on the slide plate are schematically shown in FIGS. 7A, 7B and 7C, for example, when draw forming for an oil pan for an automobile is performed. A slide plate 40 is shown as the x and y coordinates in each of the drawings. For example, when forming is started, the upper die first reaches the drainage portion of the oil pan to form the drainage portion, and therefore the force generated at the portion is applied on the fourth quadrant of the x and y coordinates. When the forming proceeds, the oil pan portion is formed, and the slide plate receives the large forces w2 and w3 from the second quadrant and the third quadrant of the coordinates. At this time, the force of w1 existing from the beginning becomes small, and since the large force w4 of the first quadrant is added, the composite force W of them is applied on the third quadrant. When the forming further proceeds, the forces w2 to w4 become small and the force w5 is added, then the composite force is approximately on the x-axis and works to the right from the y-axis.
The way how the forces and the composite force explained here are exerted, the magnitude of the forces and the composite force explained here will change depending on the shape of the work piece and the speed at which the dies move, but it can be generally said that the location and the magnitude of the composite force worked on the slide plate change as the pressing proceeds.
The inventors have proposed a press forming machine which makes a slide plate keep preferable positional relationship with respect to a lower support stand throughout press forming. In the press forming machine, the slide plate is driven and pressurized by a plurality of driving sources, and displacement measuring devices for measuring displacement of the slide plate are provided near the portion where each of the driving sources engages with the slide plate. The displacement of the slide plate is measured with the displacement measuring devices at each stage of the press forming, and a driving signal is supplied to each of the driving sources so that preferable positional relationship of the slide plate with respect to the lower support stand can be maintained.
The press forming machine is shown in the front view in FIG. 5, and in the plan view in FIG. 6. In FIG. 6, the upper support plate is shown with part of it being removed. A lower support stand 10 of the press forming machine is fixed on a floor surface, and an upper support plate 30 is held by support pillars 20 placed upright on the lower support stand. A slide plate 40 capable of reciprocating along the support pillars 20 is provided between the lower support stand 10 and the upper support plate 30, and the forming space is provided between the slide plate and the lower support stand. In the forming space, a stationary die (lower die) for pressing is mounted on the lower support stand, a movable die (upper die) corresponding to the stationary die is mounted on the undersurface of the slide plate, and for example, a plate to be formed is placed between the both dies to perform forming.
Five components each combining a servomotor and a speed reduction mechanism are mounted on the upper support plate 30 as drive sources 70. The drive shaft 71 extending downward from each of the driving sources engages with each engaging portion 72 on a top surface of the slide plate 40 through a through hole 92′ provided in a reference plate 90′. For example, a ball screw is attached to the drive shaft to convert the rotation to the vertical motion, and the drive shafts move the slide plate up and/or down by the rotation of the servo motor.
Each of displacement measuring devices 80 is provided near each of the engaging portions 72. A magnetic scale 81 of the displacement measuring device 80 is mounted on the reference plate 90′, and a magnetic sensor 82 of the displacement measuring device is supported at a support frame mounted on each of the engaging portions 72. Here, the reference plate 90′ is held at the same position regardless of displacement of the slide plate 40. When the slide plate 40 is driven by the driving sources 70, the displacement of each of the engaging portions can be measured by the displacement measuring device 80.
As the forming proceeds, the forces working on the slide plate change as in FIGS. 7A, 7B and 7C explained above. With the change, the load onto the driving sources 70 changes. The positional relationship between each region of the movable die corresponding to each of the driving sources and the stationary die is not uniform. Some of them press down the slide plate 40 fast, and others slowly descend to press down the slide surface 40. The lead and the delay of each of the driving sources are measured by the displacement measuring devices 80, then they are sent to a control, and a drive pulse signal to each of the driving sources 70 is adjusted so that the displacement measured by each of the displacement measuring devices 80 becomes a desired value, namely, all the regions of the engaging portions on the slide plate become, for example, horizontal to each other.
Since the slide plate can descend while keeping the preferable positional relationship with respect to the lower support stand, according to the press forming machine explained here, uniform forming is made possible even when the location on which a reaction force is applied from a work piece changes during forming.
However, as shown by the dashed line in FIG. 5, deformation of the slide plate varies according to location. When the location at which the displacement measuring device is mounted is away from the portion where the drive shaft of the drive source engages with the slide plate, the displacement of the slide plate which should be measured at the location of the drive shaft is measured at the position in which the displacement measuring device is mounted, that is away from the location of the drive shaft. Consequently, the displacement of the slide plate measured by the displacement measuring device does not sometimes express the real displacements of the slide plate portions with which the drive shafts engage.